Shielding gases for mag-welding of galvanized steel sheets and welding method using the same

ABSTRACT

A shielding gas for MAG welding wherein a carbon steel solid wire is used for lap fillet welding of a galvanized steel sheet; wherein the shielding gas is a mixed gas composition consisting of 8 to 15% by volume of oxygen, 20 to 30% by volume of carbon dioxide, and residual % by volume of argon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a MAG (metal active gas) welding methodof a galvanized steel sheet wherein corrosion proofing is provided onthe surface of the steel sheet.

Priority is claimed on Japanese Patent Application No. 2005-091198,filed Mar. 28, 2005, the content of which is incorporated herein byreference.

2. Description of the Related Art

In the fields of components and equipment pieces of associated withand/or used for automobiles, business equipment and the like, agalvanized steel sheet is widely used due to its excellent corrosionresistance and luster thereof. The galvanized steel sheet is a sheet inwhich galvanization is conducted on an iron base surface of a steelsheet.

However, there is a problem in that many pits and blowholes aregenerated when MAG welding is conducted on a galvanized steel sheet(s).The reason is that, since zinc has a melting point (419° C.) and boilingpoint (907° C.), which are lower than those of steel having a meltingpoint (1535° C.) and boiling point (2750° C.), when zinc enters in amolten pool while welding is conducted, zinc vapor generated from thezinc is caught in the molten pool together with air, and blowholes areformed when said air and zinc vapor fail to reach the surface before thesolidifying process of the welded metal is completed. When such pits areproduced, a problem arises in that strength which should be achieved bythe welding inherently cannot be obtained. Therefore, when pits aregenerated, repair of the portion where pits are generated is conducted.Furthermore, when a galvanized steel sheet is welded at high speed,further large numbers of pits, blowholes and the like are formed, andthis is unpreferable.

In order to solve the problems, the following methods and the like havebeen conducted in general.

(1) Zinc is removed mechanically in advance from a line on which weldingshould be conducted. However, in the method, the number of steps forwelding increase due to the addition of steps for removing zinc from theline, and this causes a problem in that the cost increases due to theaddition of the steps.

(2) Occurrence of pits is prevented such that a zinc vapor generated isprevented from moving to the surface of the metal by using, for example,a welding wire, which comprises increased amounts of Si, Mn and thelike, for welding a galvanized steel sheet in order to increase theviscosity of the molten metal. On the other hand, occurrence of pits andblowholes is prevented such that, for example, a welding wire comprisingdecreased amounts of Si, Mn and the like is used for welding agalvanized steel sheet in order to decrease the viscosity of the moltenmetal so that a zinc vapor rise to the surface of the metal easily.However, in fact, such wires do not provide excellent effects forpreventing the generation of pits and/or blowholes.

(3) A method is proposed, for example, in Japanese Patent No. 2668125,wherein generation of pits and blowholes is prevented by using a mixedgas consisting of oxygen, carbon dioxide, and argon, which occupies theresidual volume of the mixed gas, for welding. The mixed gas is used inorder to prevent an occurrence of a zinc vapor by the effect ofoxidation of zinc caused by oxygen. Furthermore, a method is proposedwherein generation of pits and blowholes is prevented due to the effectthat surface tension of the molten metal is decreased due to oxygen andgases pores included in a molten metal rise to the surface easily.However, the disclosed welding speed thereof is only 120 cm/min.

Accordingly, the purpose of the present invention is to provide ashielding gas which does not generate pits and blowholes in a weldedportion when a lap fillet welding or the like of a galvanized steelsheet is conducted in a MAG welding method.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a shielding gas for MAGwelding wherein a carbon steel solid wire is used for lap fillet weldingof a galvanized steel sheet; wherein the shielding gas is a mixed gascomposition consisting of 8 to 15% by volume of oxygen, 20 to 30% byvolume of carbon dioxide, and residual % by volume of argon.

It is preferable that the composition of the mixed gas is 8.5 to 12% byvolume of oxygen, 20 to 30% by volume of carbon dioxide, and residual %by volume of argon.

A second aspect of the present invention is a method for MAG welding ofa galvanized steel sheet, wherein a mixed gas composition consisting of8 to 15% by volume of oxygen, 20 to 30% by volume of carbon dioxide, andresidual % by volume of argon, is used as a shielding gas to conduct alap fillet welding for a galvanized steel sheet by using a carbon steelsolid wire.

It is preferable that the composition of the mixed gas is 8.5 to 12% byvolume of oxygen, 20 to 30% by volume of carbon dioxide, and residual %by volume of argon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view which shows a welding state of a concreteexample.

FIG. 2 is a photograph which shows a cross section of a bead of aconcrete example.

FIG. 3 is a photograph which shows a cross section of a bead of aconcrete example.

FIG. 4 is a photograph which shows a cross section of a bead of aconcrete example.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is described in detail.

In the present invention, a mixed gas composition consisting of 8 to 15%by volume of oxygen, preferably 8.5 or more and/or 12% by volume orless, 20 to 30% by volume of carbon dioxide, and residual % by volume ofargon, that is 72 to 55% by volume of argon, is used as a shielding gas.The shielding gas of the present invention can be used for a MAG weldingmethod wherein welding such as lap fillet welding is conducted for agalvanized steel sheet and a carbon steel solid wire is used preferably.

Here, when an oxygen concentration in the shielding gas is less than 8%by volume, pits and bow holes tend to be produced to decrease thestrength of a welded portion. When the oxygen concentration in theshielding gas exceeds 15% by volume, pits and blowholes tend to beproduced and the strength of a welded portion tend to becomeinsufficient. When a concentration of carbon dioxide is less than 20% byvolume, pits and blowholes tend to be produced and the strength of awelded portion tend to become insufficient. When the concentration ofcarbon dioxide is over 30% by volume, pits and blowholes tend toproduced and the strength of a welded portion tend to becomeinsufficient.

Discharge flow rate of the shielding gas from a welding torch is set inthe range of 15 to 25 liters/minute in general, but it is not necessaryin the invention to limit this rate in this range as long as theshielding gas can cover the surface of a molten pool.

Any kinds and conditions of a galvanized steel sheet can be used as abase metal which is welded in the present invention, and it is notparticularly limited. Furthermore, in addition to the case that two ormore sheets of galvanized steel are welded using the shielding gas ofthe present invention, such a case that the shielding gas of the presentinvention is used for welding a galvanized steel sheet to another typeof a steel sheet(s) can also be included in the scope of the presentinvention.

Furthermore, as a carbon steel solid wire usable in the presentinvention, a welding wire such as YGW-17, YGW-18 and YGW-19, which isprovided according to JIS (Japanese Industrial Standard) Z-3312 “solidwire used for MAG welding of a mild steel and a high strength steel” canbe used in the present invention. Conditions for lap fillet welding arenot particularly limited in the present invention, since lap filletwelding is a well-known method and any general conditions thereof areapplicable in the present invention.

The MAG welding method is also a well-known welding method wherein arcwelding is conducted using an active gas such as carbon dioxide as ashielding gas. The method can be conducted using a commerciallyavailable MAG welding machine. In the present invention, theaforementioned mixed gas including three kinds of gases is used as theshielding gas.

Welding arc voltage in MAG welding is generally about 14 to 36 V andwelding current, which is used to provide a heat required for welding,is generally about 40 to 350 A. In the present invention, suchconditions can be used but are not limited only thereto.

Due to the use of the shielding gas of the present invention, occurrenceof defects such as blowholes and pits in a welding portion can bereduced. Furthermore, the welding speed can be increased. For example, awelding speed of 125 cm/min or more is possible. For example, byincreasing the welding current, the welding speed can be increased toabout 200 cm/min without occurrence of blowholes and pits. It ispossible to further increase the welding speed to, for example, 250cm/min or 300 cm/min or more, by adjusting the welding conditions.Specific examples of the welding speed of the invention are for example,0 to 250 cm/min, 100 to 200 cm/min, 100 to 190 cm/min and the like, butthe rate is not only limited thereto.

Hereinafter, concrete examples of the present invention are described toclarify the functional effects of the present invention.

Concrete examples and comparative examples of the shielding gas used forMAG welding of a galvanized steel sheet of the present invention areexplained below for the purpose of illustration. Furthermore, in orderto confirm the characteristics and effects of the shielding gas of thepresent invention, certain verification tests to clarify thecharacteristics were conducted in the following welding examples.

EXAMPLE 1

As shown in FIG. 1, two galvanized steel sheets 1 having a sheetthickness of 2.3 mm were used for Example 1. A clearance gap t betweenan upper galvanized sheet and a lower galvanized sheet was set to 0 mm,and an slope angle α of a torch 2 was provided at 30°. Then, MAG weldingwas conducted so that a length of a welded portion became 250 mm whilewelding speed was varied, and occurrence and state of pits and blowholeswere confirmed.

MAG weldings were conducted such that a mixed gas consisting of argongas, carbon dioxide gas and oxygen gas was used and the composition ofthe mixed gas (volume %) was varied. Here, evaluations regarding gaseswherein the oxygen concentration in the shielding gas was more than 0%by volume and less than 7.5% by volume or exceeds 15% by volume, andgases wherein the concentration of carbon dioxide was less than 20% byvolume or exceeds 30% by volume were omitted since it was already provedby the inventors that pits and blowholes tend to be produced and thestrength of a welded portion tend to become insufficient.

For comparison, a shielding gas (Ar-20% CO₂), which is widely usedgenerally for MAG welding and contains 80% by volume of Argon and 20% byvolume of CO₂, was used.

(Welding conditions)

Welding method: consumable electrode welding, pulsed arc welding

Base metal: galvanized steel sheet

Sheet thickness: 2.3 mm

Joint form: lap fillet welding

Welding wire: a wire according to JIS YGW-17, diameter 1.2 mm

Contact tube-to-work distance: 15 mm

Torch slope angle: 30°

Welding speed: 100 to 175 cm/min

Arc voltage: 22.5 to 24.5 V

Welding current: 225 A

In Example 1, the welding speed was varied while the welding current andtotal feed of wire were maintained at fixed values.

Results obtained from the evaluations are shown in Tables 1 and 2 andFIG. 2.

In Table 1, the number of pits observed at the welded portion is shown,and a case wherein the number of pits observed was two or less isdetermined as acceptable.

In Table 2, a center portion of a bead was cut and the cross section ofthe bead is observed to evaluate whether or not blowholes weregenerated, and a case wherein blowholes were not generated wasdetermined as acceptable.

FIG. 2 is a photograph of the aforementioned cross section of the bead.TABLE 1 Welding speed (cm/min) 100 125 150 175 Acceptability Ar—20% CO₂0 1 3 34 Non-acceptable Ar—20% CO₂—7.5% O₂ 0 0 3 13 Non-acceptableAr—20% CO₂—8% O₂ 0 0 1 3 Non-acceptable Ar—20% CO₂—8.5% O₂ 0 0 0 2Acceptable Ar—20% CO₂—12% O₂ 0 0 0 0 Acceptable Ar—20% CO₂—15% O₂ 0 0 18 Non-acceptable Ar—30% CO₂—8% O₂ 0 0 3 8 Non-acceptable Ar—30% CO₂—8.5%O₂ 0 1 2 0 Acceptable Ar—30% CO₂—12% O₂ 0 0 0 1 Acceptable Ar—30%CO₂—15% O₂ 0 1 3 0 Non-acceptable(Units: number of pits)

TABLE 2 Welding speed (cm/min) 100 125 150 175 Acceptability Ar—20% CO₂◯ X X X Non-acceptable Ar—20% CO₂—7.5% O₂ ◯ ◯ X X Non-acceptable Ar—20%CO₂—8% O₂ ◯ ◯ ◯ X Non-acceptable Ar—20% CO₂—8.5% O₂ ◯ ◯ ◯ ◯ AcceptableAr—20% CO₂—12% O₂ ◯ ◯ ◯ ◯ Acceptable Ar—20% CO₂—15% O₂ ◯ ◯ X XNon-acceptable Ar—30% CO₂—8% O₂ ◯ ◯ X X Non-acceptable Ar—30% CO₂—8.5%O₂ ◯ ◯ ◯ ◯ Acceptable Ar—30% CO₂—12% O₂ ◯ ◯ ◯ ◯ Acceptable Ar—30%CO₂—15% O₂ ◯ ◯ ◯ ◯ Acceptable◯: No occurrence of blowholes,X: Occurrence of blowholes

In Example 1, the welding current was fixed at 225 A. When the weldingcurrent is fixed at 225 A even in a case of the welding speed of 175cm/min or more, welded metal shortage may be caused, and thereforeinsufficient depth and leg length may be caused. The welding speed canbe increased due to an increase of welding current.

EXAMPLE 2

In Example 2, MAG welding was conducted similar to in Example 1 exceptthat the torch slope angle, welding speed, arc voltage, and weldingcurrent were changed as follows. Clearance gap t between an upper sheetand a lower sheet was set to 1 mm.

(Changed Welding Conditions)

Torch slope angle: 45°

Welding speed: 130 to 190 cm/min

Arc voltage: 24 to 27.5 V

Welding current: 280 to 350 A

Since the amounts of welded metal are reduced when the welding speed isincreased but the welding current was maintained, welding was conductedsuch that the welding speed was increased while the welding current wasalso increased to prevent the occurrence of insufficient amounts ofwelded metal.

The results are shown in Tables 3 and 4 and FIGS. 3 and 4.

The results of Table 3 and FIG. 3 represent comparisons between a casewherein a shielding gas consisting of 20% by volume of carbon dioxide,12% by volume of oxygen and residual % by volume of argon was used and acase wherein a general shielding gas consisting of 20% by volume ofcarbon dioxide and residual % by volume of argon, wherein gaseous oxygenwas not included therein, was used.

The results of Table 4 and FIG. 4 represent comparisons between a casewherein a shielding gas consisting of 30% by volume of carbon dioxide,12% by volume of oxygen and residual % by volume of argon was used and acase wherein aforementioned general shielding gas was used.

Evaluation and decision regarding whether blowholes were generated ornot were conducted such that a case in which blowholes were notgenerated was represented with a sign “◯” and a case in which blowholeswere generated was represented with a sign “×”. TABLE 3 Gas of theWelding present invention Conventional gas speed Pits Pits (cm/min)(number of pits) Blowholes (number of pits) Blowholes 130 0 ◯ 0 ◯ 150 0◯ 0 ◯ 170 0 ◯ 0 X 190 0 ◯ 0 X

TABLE 4 Gas of the Welding present invention Conventional gas speed PitsPits (cm/min) (number of pits) Blowholes (number of pits) Blowholes 1300 ◯ 0 ◯ 150 0 ◯ 0 ◯ 170 0 ◯ 0 X 190 0 ◯ 0 X

When the welding current was increased to the maximum value of 350 A, itwas possible to increase a welding speed at 190 cm/min or more. However,in this case, the welding speed of 190 cm/min was the maximum ratesubstantially when the current is 350 A. That is, there was apossibility that insufficient throat, undercut, and/or bead convexitywere generated when the welding speed exceeds 190 cm/min. However, byconsidering the results of Example 1, it is clear that it is possible toovercome such problems by increasing the welding current.

Results of Examples 1 and 2 were collated, and the relationship betweenthe composition of shielding gases and the occurrence of generated pitsand blowholes is shown in Table 5. TABLE 5 Pits Blowholes Ar—20% CO₂ X XAr—20% CO₂—7.5% O₂ X X Ar—20% CO₂—8% O₂ Δ Δ Ar—20% CO₂—8.5% O₂ ◯ ◯Ar—20% CO₂—12% O₂ ⊚ ◯ Ar—20% CO₂—15% O₂ Δ (which is nearly X) X Ar—30%CO₂—8% O₂ Δ (which is nearly X) X Ar—30% CO₂—8.5% O₂ ◯ ◯ Ar—30% CO₂—12%O₂ ◯ ◯ Ar—30% CO₂—15% O₂ Δ ◯

As shown in Table 5, it is clear that generation of pits and/orblowholes can be suppressed due to the use of a mixed gas consisting of8 to 15% by volume of gaseous oxygen, preferably 8.5 to 12% by volume,and 20 to 30% by volume of carbon dioxide, and argon, which occupies theresidual volume, as a shielding gas.

Here, in Table 5, a sign “⊚” in the column for pits represents that pitsare not generated at all; a sign “◯” represents that some pits aregenerated but the number of pits is smaller than the reference number; asign “Δ” represents that pits are generated and the number thereof issomewhat larger than the reference number; and a sign “×” representsthat a lot of pits are generated. Furthermore, a sign “◯” in the columnfor blowholes represents that blowholes are not generated at all; a sign“Δ” represents that blowholes are generated at the welding speed of 175cm/min or more; and a sign “×” represents that blowholes are generatedat the welding speed of 125 cm/min or 150 cm/min or more.

While preferred embodiments of the invention have been described andillustrated above, it should be understood that these are exemplary ofthe invention and are not to be considered as limiting. Additions,omissions, substitutions, and other modifications can be made withoutdeparting from the spirit or scope of the present invention.Accordingly, the invention is not to be considered as being limited bythe foregoing description, and is only limited by the scope of theappended claims.

INDUSTRIAL APPLICABILITY

In the present invention, generation of pits and blowholes can beprevented by using a shielding gas for MAG welding, wherein the gas is amixed gas of three kinds consisting of 8 to 15% by volume of oxygen,preferably 8.5 to 12% by volume, 20 to 30% by volume of carbon dioxide,and residual % by volume of argon. Furthermore, due to the effect of theshielding gas, the welding speed can be increased, for example, awelding speed of 125 cm/min or more is possible.

1. A shielding gas for MAG welding wherein a carbon steel solid wire isused for lap fillet welding of a galvanized steel sheet; wherein theshielding gas is a mixed gas composition consisting of 8 to 15% byvolume of oxygen, 20 to 30% by volume of carbon dioxide, and residual %by volume of argon.
 2. The shielding gas according to claim 1, whereinthe composition of the mixed gas consists of 8.5 to 12% by volume ofoxygen, 20 to 30% by volume of carbon dioxide, and residual % by volumeof argon.
 3. A method for MAG welding of a galvanized steel sheet,wherein a mixed gas composition consisting of 8 to 15% by volume ofoxygen, 20 to 30% by volume of carbon dioxide, and residual % by volumeof argon, is used as a shielding gas to conduct lap fillet welding for agalvanized steel sheet by using a carbon steel solid wire.
 4. The methodaccording to claim 3, wherein the composition of the mixed gas consistsof 8.5 to 12% by volume of oxygen, 20 to 30% by volume of carbondioxide, and residual % by volume of argon.
 5. The method according toclaim 3, wherein a welding speed of the lap fillet welding is 125 cm/minor more.